The Twin has seen a number of revisions since its introduction, both internal and external, with its designs sometimes varying greatly from one year to the next.[1] Several variations on the amp's original design have been produced through the years, including the Twin Reverb, the Super Twin, the Twin Reverb II, and the Twin Reverb '65 Reissue. The Cyber Twin, which combined a tube amp with a computerized processor, was introduced in January 2001.[2]

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After the preceding looks of the early 50's (TV front from 1950 to 51/2; wide panel '52–54), Leo Fender changed the cabinet design again, this time opting for no extra wood on the front of the amp, except for the narrow top and bottom panels that hold the baffle board to the cabinet.

The early models of the larger "narrow-panel" tweeds are also remarkable for their refined electronics whose circuit design incorporated dual 5U4 rectifiers in the Twin and Bassman models, another improvement given Fender's quest for a louder, cleaner amplifier. The entire line of Fender amplifiers from 1955 to 1959 (later for smaller models and Bassman) was uniform in this look—tweed or "airline linen" covering with a maroon with gold stripe woven saran grill cloth. The 1×12 Deluxe-Amp, the 1×15 Pro-Amp and the 3×10 Bandmaster are exceptional in dynamics and tone.

Like its predecessors, the narrow panel tweed Fender amplifiers used Jensen Alnico V Concert Series Speakers, for the most part, although a JBL 15" speaker was available on special order for the Pro model.

It was modified in 1958 for more volume, switching to four 5881 power tubes[8] with the more efficient long-tailed pair phase inverter for a power increase to 80W. This "hi-powered," tweed-covered design continued into early 1960, after the other Professional Series of Fender amplifiers had made the transition to the modern brownface design.

The exact plight of the Twin-Amp during the months between January and May 1960 however, remains open to considerable speculation, debate and study. The prevailing explanation is that production was temporarily interrupted during these months as they coincide with the debut of Fender's new "flagship" or top of the line amplifier, the Fender Vibrasonic-Amp. Nonetheless, Fender Musical Instrument Co. kept the image of the Twin-Amp before potential consumers during this short period. The image of the Twin-Amp in the 1960 Fender Catalog has been the subject of considerable scrutiny.

The re-emergence of the Twin-Amp in mid 1960 revealed a new aesthetic design that would become prominent among Fender's top of the line amplifiers, with the exception of the Vibrasonic-Amp. By 1961, the Bandmaster, the Bassman and the newly debuted Showman were all covered in the new look exemplified by the late 1960 Twin-Amp: blonde tolex and maroon or "oxblood" grille cloth. The Twin-Amp of this period (late 1960–1963) was manufactured with a variety of speakers including Jensen, Oxford and JBL designs. This variation lends support to the idea that the 80-watt circuit was beyond the power handling capacity of the speakers of the late 1950s. After a slight change in appearance, from the rough blonde tolex and maroon grille appearance to a smooth blonde tolex with a gold grille cloth, the Twin-Amp was replaced with the Twin Reverb-Amp in 1963.

The Fender Twin Reverb is an all-tube guitar amplifier with two 12", 8-ohm speakers made by various suppliers. The more common speakers found in Fender Twin Reverbs include Jensen C12Ns, Oxford 12T6s, JBL D-120Fs (an upgrade at the factory), Altec Lansing 417-8Cs, Utahs and, in later years, CTS and Eminence models. Some Twins were fitted with EVM 12L variant speakers known as EVM 12Fs. Unlike most Electrovoice speakers, they were fitted with metal dustcovers like the JBL D-120Fs. The Twin Reverb is frequently shortened in conversation to "Fender Twin" or simply "Twin", though it should not be confused with the Tweed or Blonde era Twin amplifiers which had vastly different circuits and no reverb. The Fender Twin Reverb is considered a standard model for players seeking a clean sound, and it is especially known for the quality of its built-in spring reverb. All Twin Reverbs feature a solid-state rectifier.[9]

The Twin Reverb was first introduced during the blackface era of Fender amplifiers which was from 1963 to 1967. During this time the amplifier's output was rated at 85 watts into a 4-ohm speaker load.

The so-called blackface amplifiers have black faceplates, black tolex covering on the cabinet, and neutral to slightly amber tinted silver sparkle grill cloth with a small ribbed rectangular pattern.

Twin Reverb amplifiers came standard with "tilt back legs" which allowed the amplifier to be tilted at an angle backwards, so the speakers faced at a more upward angle, promoting better distribution of their output to an audience when placed on a low stage.

The circuit used is commonly known as the AB763 circuit. Fender Twin Reverb amplifiers use four output tubes, of the 6L6GC type.[10] They use six preamp tubes, consisting of four 7025/12AX7 types and two 12AT7 types. The 7025 is a lower-noise version of the 12AX7.

The Fender Twin Reverb has two independent channels, labeled Normal and Vibrato. The controls have black-skirted knobs numbered from 1 to 10.

The Normal channel has two inputs, a "bright" switch (which compensates for loss of brightness through the volume control when the control is set lower than about "6" on its 1-10 scale), a volume control, treble, middle and bass tone controls.

The Vibrato channel has a duplication of the same controls as the normal channel, plus the addition of reverb, vibrato speed and intensity controls. Reverb is accomplished with a tube/transformer driven low impedance spring reverb tank (made by the Hammond company) or its spin-off company Accutronics.[11] The addition of the reverb circuit requires an additional "gain" stage in the preamp, and as such, the Vibrato channel is capable of a bit more distortion when the volume is set high.[citation needed] Vibrato (tremolo) is accomplished with what is known as a "vibrato bug" by Fender amplifier repairmen. The vibrato bug is a four-wire device consisting of a neon tube and light-dependent variable resistor, packaged in a short section of black tubing. It resembles a water bug, with slender wispy legs. The tube generated vibrato oscillator pulsates the neon light, which varies the resistance on the resistive element. That variable resistance is applied to the audio signal on the Vibrato channel, creating a pulsating increase and decrease of that channel's volume. The speed controls varies the rate of the oscillator. The depth control limits the amount of application the variable resistor has on the audio signal.

The front panel also has a bright red pilot light lens (better known as "pilot lamp jewel" for its multiple triangular-shaped facets), covering the pilot light (made by the Chicago Miniature Lamp Co.). Other colour schemes (amber, white, green, purple and blue) are also available.

Rear panel controls include a 2.5-amp "slo-blo" fuse, an accessory AC outlet, an earth ground selection switch, on/off and standby switches. Additionally there are two speaker output jacks, and jacks for footswitches to activate/deactivate the reverb and vibrato effects.

The black faceplates of the "blackface" era Twin Reverb will say either "Fender Electric Instruments" (FEI) or "Fender Musical Instruments", (FMI) depending on the date of manufacture. Those units made before the CBS take over of Fender in 1965, will be marked Fender Electric Instruments, and be worth a bit more on the collectors market.

In 1968 the Fender amplifier line switched from the original black faceplate to a new brushed aluminum faceplate with light blue labels (except the Bronco, which has red) and changed the color of the grillcloth from silver grey to silver with sparkling blue threads embedded within it, ushering in the Silverface era. Other blackface cosmetic features were retained. The first silverface Twins used the blackface AB763 circuit until May 1968, when Fender switched to the AC568. Since the tube complement was the same, Fender just used up their stock of printed tube charts saying AB763 until they ran out.[12]

The earliest silverface Twin Reverbs, along with all other silverface models, had an aluminum frame (trim) surrounding the sparkling blue grillcloth from late 1967 to 1969. Early silverface amplifiers made between 1967 and 1968 had black lines on the brushed aluminum faceplate, still retaining the '60s "tailed" design on the amp logo, installed on the upper left side of the grillcloth. This feature was offered on models produced prior to the "tailless" period in 1973. Some later models came with an unusual silver grillcloth with sparkling orange threads ("orange sparkle"); a black grillcloth was even fitted on some production runs. During the silverface period, the Twin Reverb's chassis and AA769 circuit was shared by the Dual Showman Reverb, effectively producing the Twin Reverb in a head form, although the matching speaker cabinet for the Dual Showman Reverb sported a pair of 15-inch JBL speakers rather than 12-inch.

In 1972 a master volume was added, then in late '75 it was fitted with a push pull "boost" or acentric potentiometer in the master volume position. The rating of the amplifier's output power was upgraded to 100 watts and between 1977–1982 an ultra linear output transformer was used, increasing the power to 135 watts. A Hum Balance was added at this time

In 1982 the Twin Reverb II was introduced to compete with Mesa Boogie's very successful products; it had an output rating of 105 watts and a return to Blackface era cosmetics albeit with a different, channel-switching cascading gain circuit with a more modern mid-range voicing and without the 'traditional' vibrato. The Twin Reverb II was produced until 1986, and was available as a head (amplifier alone) or 2×12 combo. The circuits for this series of 1982 Fender amplifiers were designed by amp guru Paul Rivera; the 60w Concert amplifier was by far the most successful in terms of sales, but all models in this range and the next are now becoming highly sought-after by collectors and players.

The Twin Reverb II was replaced by The Twin, commonly referred to, though incorrectly as, the "Evil Twin" (Fender has only referred to the "TWIN AMP" or the '95 Twin Amp as being the "Evil Twin", not the red knob Twin) due to the addition of both a separate gain channel and a switchable overdrive on the clean, in 1987. It featured a dual output switch, allowing the user to select either 25 watts or 100 watts of output. The Twin was designed for maximum versatility for live or studio use with features like speaker jacks with selectable impedance, Low-z/XLR output, a "Power Amp Thru" feature that bypassed the preamp circuit and a buffered effects loop. Both clean and overdrive channels could be activated simultaneously and reverb could be turned on/off for each channel. Earlier Twin Reverb amps were known among musicians to be best suited for loud, "clean" tones. The "Red Knob Twin" was made until 1994; During 1994 it was slghtly modified and the knobs were changed from red to black, keeping the same shape. It has since been succeeded by the first version "Twin Amps" of Fender's Protube line in 1995. This was the first in the series to use PCB (printed circuit boards) instead of PTP (point to point) wiring. All subsequent models and reissues have used PCBs.

This version has Blackface cosmetics, two inputs, no vibrato, two channels with gain select on the floor pedal to give it two sounds per channel and an effects loop. This model is referred to as the "Evil Twin".[13] In 2002 it was replaced by a new Pro Tube "Twin Amp" including tremolo.

This version is very similar in cosmetics with the previous one but it introduces, for the first time, a full tube tremolo in a modern twin amp. The front panel contains (left to right): a single input, clean channel controls (volume, bright switch, treble, middle, bass), distorted channel controls (volume, treble, middle, bass, master volume, channel select), and shared controls (reverb, tremolo speed and intensity, presence). The rear panel contains power and stand by switches, output power switch (100W or 25W), effect send level, effect loop switch, return level, preamp out, power amp in, footswitch jack, tube bias adjustment and balancing, external speaker jack, main speaker jack. The footswitch has four buttons that allow to select the channel, bypass or engage the effect loop, switch the tremolo on/off, switch the reverb on/off. The tremolo is a tube bias tremolo. Curiously, it can be switched on through the footswitch only. The amplifier features two 12 inch Eminence special design loudspeakers in parallel (8ohms each, 4 ohms total load). There is no output impedance switch and the total impedance of the main plus external speaker has to be 4 ohms.[14]

In 1992 Fender introduced the '65 Reissue,[15] with blackface cosmetics and circuitry, an output rating of 85WRMS and 8ΩJensen C-12K speakers. It should be noted, however, that the '65 Reissue is constructed with a printed circuit board and new 1/4" phone jacks for the footswitch pedal replacing the older RCA design of the vintage originals. Fender's original intention was to issue only 300 of this model, but it was continued due to its success in the market. The '65 Reissue is also available with a single 15-inch speaker, called the '65 Twin Custom 15.[16]

In 2013, Fender introduced a redress of the '65 reissue dubbed the '68 Custom Twin Reverb. It came with silverface cosmetics, reverb and vibrato on both channels, altered negative feedback circuitry, Celestion Type V speakers and a "Custom" channel (channel 1) which utilizes a modified tone stack derived from the Fender Bassman. The "vintage" channel (channel 2) is meant to be the typical Twin sound. The output remains at 85W, unlike the original Twin Reverb of 1968.[17]

In 2012, Fender released a series of three tweed-style amplifiers as an artist signature line, endorsed by Eric Clapton. One of these amps is the Twinolux, a modified version of the 5E8 Tweed Twin that removed one channel, and added a tremolo effect, with an overall 40W output.[18] Listed at $3000, it was praised by Vintage Guitar for its "exceptional" tones, both clean and driven.[19]